Distances.pairwise fails on TrackedArray

```julia
julia> using Distances, Flux

julia> pairwise(SqEuclidean(), param([1 2; 3 4]), dims = 2)
ERROR: MethodError: no method matching Float64(::Tracker.TrackedReal{Float64})
Closest candidates are:
Float64(::Real, ::RoundingMode) where T<:AbstractFloat at rounding.jl:194
Float64(::T<:Number) where T<:Number at boot.jl:718
Float64(::Int8) at float.jl:60
...
Stacktrace:
[1] convert(::Type{Float64}, ::Tracker.TrackedReal{Float64}) at ./number.jl:7
[2] setindex!(::Array{Float64,1}, ::Tracker.TrackedReal{Float64}, ::Int64) at ./array.jl:766
[3] sumsq_percol(::TrackedArray{…,Array{Float64,2}}) at /home/user/.julia/packages/Distances/jwhuc/src/common.jl:107
[4] _pairwise!(::Array{Tracker.TrackedReal{Float64},2}, ::SqEuclidean, ::TrackedArray{…,Array{Float64,2}}) at /home/user/.julia/packages/Distances/jwhuc/src/metrics.jl:600
[5] #pairwise!#2(::Int64, ::typeof(pairwise!), ::Array{Tracker.TrackedReal{Float64},2}, ::SqEuclidean, ::TrackedArray{…,Array{Float64,2}}) at /home/user/.julia/packages/Distances/jwhuc/src/generic.jl:184
[6] #pairwise! at ./none:0 [inlined]
[7] #pairwise#4(::Int64, ::typeof(pairwise), ::SqEuclidean, ::TrackedArray{…,Array{Float64,2}}) at /home/user/.julia/packages/Distances/jwhuc/src/generic.jl:213
[8] (::getfield(Distances, Symbol("#kw##pairwise")))(::NamedTuple{(:dims,),Tuple{Int64}}, ::typeof(pairwise), ::SqEuclidean, ::TrackedArray{…,Array{Float64,2}}) at ./none:0
[9] top-level scope at REPL[2]:1
```

Could you please make it work? I think the problem is [here](https://github.com/JuliaStats/Distances.jl/blob/5dbf9b837989d5c9c6a78b918ca2518c6e4ac313/src/common.jl#L104), but I don't know how to fix it (probably needs to be done in Distances.jl?)

Thanks